A conventional strain sensor using a strain sensitive resistor is explained while referring to the drawing. FIG. 18 is a sectional view of a conventional strain sensor. On a metal substrate 1, an electric insulator layer 2 of crystallized glass enamel layer is provided. A strain sensitive resistor 3 is coupled to the metal insulator layer 2, and an overcoat layer 4 is applied, and a load detecting device is composed. A vehicle suspension using such load detecting device is disclosed in Japanese Patent Publication No. 2929757. However, the surface of crystallized glass is rough and is not small, and is large in fluctuations of resistance value as disclosed in Japanese Patent Application Laid-Open No. 6-137805.
FIG. 19 is a partial sectional view of crystallized glass before baking. The crystallized glass before baking is known to be composed of glass powder particles 5. The glass powder particles 5 are baked at specified temperature, and an electric insulator layer 2 is formed.
FIG. 20 is a magnified sectional view of crystallized glass. In FIG. 20, fine asperities are left over on the surface 6 of electric insulator layer 2, and voids 7 are included inside. Such voids 7 are generally contained in a ceramic electronic component. Not having direct effects on reliability of products, such voids may lead to characteristic fluctuations and other problems of products. Such problems are explained below by referring to FIG. 21.
FIG. 21 is a diagram showing baking conditions when baking crystallized glass by using meshbelt conveyor furnace (MCF). In FIG. 21, the axis of abscissas denotes the time and the axis of ordinates represents the temperature. The sample is gradually heated in the MCF from room temperature until once reaching a specified peak temperature of about 900° C., and is gradually cooled to room temperature. In FIG. 21, Temp1 on the Y-axis shows the softening temperature of crystallized glass, and Temp2 corresponds to the crystallizing temperature.
In the case of a general noncrystalline glass, the glass is softened more and more when reaching higher temperature by exceeding the softening temperature (glass softening starting temperature, generally determined by TDA) and the softening point. In the case of crystallized glass, the glass begins to be softened around the softening point (Temp1 in FIG. 21), but the glass is crystallized when reaching the softening temperature (Temp2 in FIG. 21). Accordingly, the softening temperature range of crystallized glass ranges between softening temperature at Temp1 and crystallizing temperature at Temp2. In FIG. 21, the time corresponding to Temp1 and Temp2 is Time1 and Time2 on the X-axis. In other words, the crystallized glass begins to be softened at Time1 (corresponding to softening temperature at Temp1), and is crystallized and solidified at Time2 (corresponding to crystallizing temperature at Temp2). The melting temperature of crystallized glass after crystallization is over 1000° C. Accordingly, the crystallized glass after once being crystallized is not melted at peak temperature (about 900° C. in FIG. 21), and maintains its solid state. That is, in baking profile 8 in FIG. 21, in the initial stage up to Time1, the crystallized glass remains in unbaked state composed of glass powder 5 as shown in FIG. 19. From Time1 to Time2, the glass powder 5 is melted simultaneously, and after Time2, it is solidified as crystallized glass.
Thus, the crystallized glass is melted only between Time1 and Time2, and a sufficient leveling (surface smoothing) time is needed when melting the crystallized glass. Accordingly, for baking stably, it is hard to shorten the baking time. As a result, the production efficiency was low in baking process.
Further, when forming crystallized glass integrally on a metal substrate, baking shrinkage of crystallized glass occurs only in the Z-direction (thickness direction). Baking shrinkage is impeded in the XY direction (that is, the side contacting with the metal substrate). As a result, unlike various products made from general crystallized glass materials (since three-dimensional shrinkage in XYZ direction is possible, stress occurring in baking process is likely to be uniform), technical difficulty is high for optimizing the baking condition of crystallized glass in the case of such configuration. That is, in this configuration, it was a difficult problem to shorten the baking time of crystallized glass.